Add model that more closely mimics rat entorhinal modules.
Here's what distinguishes this model from the other:
- It separately models the "bump size" and the "readout resolution"
rather than using the 1/cellsPerModuleAxis for both.
- It models how a union of bumps is harder to readout than a single
bump. (In this approach, more cells become "active" than just the
union of the active cells)
- Hexagonal grids. The firing fields of a cell are arranged in a
hexagon rather than in a square.
- Each firing field is round instead of square.

Other changes:

- Fix a bug: PIUNExperiment.representationSet wasn't being properly
filled, so we weren't correctly detecting temporary convergences on
a wrong object.
- Stop seeding the object generator with the number of objects. It
should generate different objects on different runs.
- Add couple other capabilities to convergence simulation.
- Tweak the Superficial2DLocationModule so that it shares a more
common interface with the new ThresholdedGaussian2DLocationModule.